As is well known in the art, the closure of a hook switch at a subscriber station of a telephone system shunts a capacitor which lies in series with the line loop and passes the alternating ringing signal, of a low audio frequency between 20 and 50 Hz, but blocks the flow of direct current due to a continuous voltage also applied to the line at the exchange. That voltage is generally much lower than the RMS voltage of the ringing signal which may be as high as 70 V.
In commonly owned U.S. Pat. No. 4,362,908 there has been disclosed a circuit arrangement of this general type suitable for the detection of hook-switch closure in an electronic exchange. As pointed out in that prior patent, such exchanges are incapable of dealing directly with low-frequency ringing currents of the intensity needed to operate a conventional sound generator at a called subscriber station. The use of a low-pass filter with a cut-off frequency of only a few Hz, designed to separate the d-c component from the accompanying alternating signal, requires rather large capacitances which in turn provide such a filter with a large time constant greatly delaying the response of the system to closure of the hook switch. Such capacitors, moreover, cannot be conveniently incorporated in integrated circuitry.
According to the prior patent identified above, a circuit arrangement at an electronic exchange compares the durations of the positive and negative half-cycles of the ringing current whose disparity indicates the superposition of a d-c component upon the alternating signal. That system, while avoiding the drawbacks of earlier solution, still requires a certain minimum ratio of d-c level to a-c peaks in order to be fully effective.